did you not know his uncle's were inappropriate homophobes? because, well, I know my siblings, and my husband's and know which ones my daughter is going to learn inappropriate things from... so don't let her hand out with them without someone to buffer their crazy.

Did your son actually know what the term meant or was just innocently saying it because he learnt it from his uncle? If so, you may need to have a talk with him and explain that the term is not appropriate to say.